Parent Company Balance Sheet (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010	Dec. 31, 2009
Assets [Abstract]
Fixed Maturity Securities	$ 42,486.7		$ 40,035.6
Mortgage Loans	1,612.3	[1]	1,516.8
Short-term Investments	1,423.5		1,163.1
Other Assets	645.3		650.6
Total Assets	60,179.0		57,307.7
Liabilities
Short-term Debt	312.3		225.1
Long-term Debt	2,570.2		2,631.3
Other Liabilities	1,815.1		1,567.6
Total Liabilities	51,602.0		48,363.3
Common Stock	35.9		36.5
Additional Paid-in Capital	2,591.1		2,615.4
Retained Earnings	7,031.2		7,060.8
Treasury Stock	1,530.1		1,110.2
Total Stockholders' Equity	8,577.0		8,944.4	8,500.1
Total Liabilities and Stockholders' Equity	60,179.0		57,307.7
Parent Company [Member]
Assets [Abstract]
Fixed Maturity Securities	172.7		357.5
Mortgage Loans	22.5		22.9
Short-term Investments	473.9		666.7
Investments in Subsidiaries	9,704.1		9,756.4
Other Assets	650.3		550.3
Total Assets	11,023.5		11,353.8
Liabilities
Short-term Debt	2.5		225.1
Long-term Debt	1,641.0		1,617.7
Other Liabilities	803.0		566.6
Total Liabilities	2,446.5		2,409.4
Common Stock	35.9		36.5
Additional Paid-in Capital	2,591.1		2,615.4
Accumulated Other Comprehensive Income	448.9		341.9
Retained Earnings	7,031.2		7,060.8
Treasury Stock	(1,530.1)		(1,110.2)
Total Stockholders' Equity	8,577.0		8,944.4
Total Liabilities and Stockholders' Equity	$ 11,023.5		$ 11,353.8

[1] Amortized cost for fixed maturity securities and mortgage loans represents original cost reduced by repayments, write-downs from other-than-temporary declines in fair value, amortization of premiums, and accretion of discounts.